Data buses are used to transfer data between devices in computer systems. In general, a data bus can transfer only a certain amount of data in a given amount of time and thus has limited “bandwidth.” Some data buses, such as a “Universal Serial Bus” (USB), support more than one type of data transfer, each with its own bandwidth and priority requirements. USB, for example, supports two types of streaming transfers for bi-directional data, known as “isochronous” transfers and “bulk” transfers.
In the USB environment, isochronous transfers are used for higher priority data, such as packetized voice data, that must reach the intended target within a bounded period of time. Isochronous transfers therefore attempt to guarantee at least some minimum bandwidth on the bus. Bulk transfers, on the other hand, are used with lower priority data, such as normal Internet traffic, for which only a guarantee of delivery is required. Bulk transfers do not guarantee bandwidth or priority on the bus.
In a conventional computer system, each network-interfacing device places all traffic that arrives over the network into a single queue. For a USB device, such as a USB broadband modem, this means that all incoming data is placed in a single queue, regardless of the data's priority. Therefore, the delivery of higher priority data is often delayed by the presence of lower priority data in the queue. This makes it difficult to provide guaranteed bus bandwidth to the higher priority data.